Dual-Polarized Microstrip Patch Antenna and Array

ABSTRACT

A microstrip patch antenna comprising: a unit cell comprising: a plurality of layers comprising: a first laminate comprising one or more horizontal polarization feed lines and one or more vertical polarization feed lines, a second laminate comprising a radiating square patch, and a third laminate comprising a parasitic patch; and a ground plane comprising one or more polarization slots. A differential feed antenna comprising: a balun; a plurality of feed lines; and one or more polarization ports configured to excite at a plurality of locations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent claims priority to U.S. Prov. Patent App. No. 62/938,741filed on Nov. 21, 2019, which is incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported by the National Oceanic and AtmosphericAdministration under Grant NA11OAR4320072 and Grant NA16OAR4320115. Thegovernment has certain rights in this invention.

BACKGROUND

There is an interest, and practical value, in utilizing polarizationdiversity for a radar to obtain increased target information or for acommunication system to carry additional signal information withoutoccupying more frequency band. This is because frequency bands aregetting crowded in microwave frequencies due to the recent advancementin cellular communications. For example, SENSR has been started to studythe feasibility of replacing the four radar networks that service theUnited States with a single network of MPARs.

Candidates being considered for future MPARs include CPPAR and PPPAR. Tohave desired accurate weather measurements with a PPPAR or CPPAR, ahigh-performance phased array antenna with dual-polarization capabilityis required. The array antenna is required to possess matched mainbeams, high input-isolation, and low cross-polarization level atbroadside and scan angles up to 45°. The beam mismatch should be within5% of the beamwidth, the input isolation needs to be better than 40 dB,and the cross-polarization level needs to be lower than −20 dB and −40dB for alternate and simultaneous transmission, respectively. These arevery stringent requirements for antenna design and development.

Numerous methods have been previously proposed for improving the antennapattern and increasing the isolation between array elements. However,the currently available designs lack polarization purity of the antennaradiation pattern.

The antenna performance and the accuracy of weather measurement could beaffected by radome conditions (e.g., wet radome), and severalinvestigations have been conducted to illustrate these effects onantenna radiation pattern and the polarimetric biases.

Achieving a dual-polarization antenna with high polarization purity hasbeen a challenge. The novel antenna configurations of the presentdisclosure address the deficiencies of the previously proposed antennadesigns.

BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the present disclosure are hereby illustrated inthe appended drawings. It is to be noted however, that the appendeddrawings only illustrate several embodiments and are therefore notintended to be considered limiting of the scope of the presentdisclosure.

FIG. 1 is a schematic showing a layer stack up of a unit cellconstructed in accordance with the present disclosure.

FIG. 2A shows Layer 1—design parameters for port 1 and port 2transmission lines.

FIG. 2B shows Layer 1—port 1 and port 2 transmission lines.

FIG. 2C shows Layer 2—slots on the ground plane.

FIG. 2D shows Layer 3—the radiating patch and Layer 4—the parasiticpatch.

FIG. 3 is a photograph of a perspective view of a fabricated unit cell.

FIG. 4 shows the simulated and measured S-parameters of the disclosedantenna.

FIG. 5 shows the simulated active S-parameters of the disclosed patchantenna under periodic boundary conditions.

FIG. 6A shows the measured radiation pattern for φ=0° plane and H-pol.

FIG. 6B shows the measured radiation pattern for φ=90° plane and H-pol.

FIG. 6C shows the measured radiation pattern for φ=0° plane and V-pol.

FIG. 6D shows the measured radiation pattern for φ=90° plane and V-pol.

FIG. 7 is a photograph showing a 2×2 element subarray using the antennasof the present disclosure.

FIG. 8A shows the measured radiation pattern for φ=0° plane and H-pol.

FIG. 8B shows the measured radiation pattern for φ=90° plane and H-pol.

FIG. 8C shows the measured radiation pattern for φ=0° plane and V-pol.

FIG. 8D shows the measured radiation pattern for φ=90° plane and V-pol.

FIG. 9A shows the array with a φ=0° plane.

FIG. 9B shows the array with a φ=90° plane.

FIG. 10A shows the measured scan radiation pattern for H-pol and φ=0°.

FIG. 10B shows the measured scan radiation pattern for H-pol and φ=90°.

FIG. 11A shows the measured scan radiation pattern for V-pol and φ=0°.

FIG. 11B shows the measured scan radiation pattern for V-pol and φ=90°.

DETAILED DESCRIPTION

The present disclosure is directed to the design and development of ahigh-performance, dual-polarization, hybrid-aperture-coupled microstrippatch antenna and arrays of such antennas. The microstrip patch antennaachieves the required antenna performance and has the added benefits ofa low profile and low fabrication costs. Also, other microwavecomponents, such as filters, can be readily integrated into this antennaarray structure. The present disclosure describes different methods forexciting two orthogonal polarizations using microstrip patch antennas.Dual-polarized hybrid feed antennas with high polarization purity couldbe an ideal choice for MPAR applications. One of the advantages of usinga hybrid feed technique to excite the single element is increasing thegeometrical symmetry of the antenna without having a complicatedmultilayer design. Although the coupling between two polarizations andcross-polarization of the antennas excited with this method are verylow, this type of patch antenna has a very compact design.

Before describing various embodiments of the present disclosure in moredetail by way of exemplary description, examples, and results, it is tobe understood that the present disclosure is not limited in applicationto the details of methods and compositions as set forth in the followingdescription. The present disclosure is capable of other embodiments orof being practiced or carried out in various ways. As such, the languageused herein is intended to be given the broadest possible scope andmeaning; and the embodiments are meant to be exemplary, not exhaustive.Also, it is to be understood that the phraseology and terminologyemployed herein is for the purpose of description and should not beregarded as limiting unless otherwise indicated as so. Moreover, in thefollowing detailed description, numerous specific details are set forthin order to provide a more thorough understanding of the disclosure.However, it will be apparent to a person having ordinary skill in theart that the embodiments of the present disclosure may be practicedwithout these specific details. In other instances, features which arewell known to persons of ordinary skill in the art have not beendescribed in detail to avoid unnecessary complication of thedescription.

Unless otherwise defined herein, scientific and technical terms used inconnection with the present disclosure shall have the meanings that arecommonly understood by those having ordinary skill in the art. Further,unless otherwise required by context, singular terms shall includepluralities and plural terms shall include the singular.

All patents, published patent applications, and non-patent publicationsmentioned in the specification are indicative of the level of skill ofthose skilled in the art to which the present disclosure pertains. Allpatents, published patent applications, and non-patent publicationsreferenced in any portion of this application are herein expresslyincorporated by reference in their entirety to the same extent as ifeach individual patent or publication was specifically and individuallyindicated to be incorporated by reference.

As utilized in accordance with the methods and compositions of thepresent disclosure, the following terms, unless otherwise indicated,shall be understood to have the following meanings:

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one,” butit is also consistent with the meaning of “one or more,” “at least one,”and “one or more than one.” The use of the term “or” in the claims isused to mean “and/or” unless explicitly indicated to refer toalternatives only or when the alternatives are mutually exclusive,although the disclosure supports a definition that refers to onlyalternatives and “and/or.” The use of the term “at least one” will beunderstood to include one as well as any quantity more than one,including but not limited to, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30,40, 50, 100, or any integer inclusive therein. The term “at least one”may extend up to 100 or 1000 or more, depending on the term to which itis attached; in addition, the quantities of 100/1000 are not to beconsidered limiting, as higher limits may also produce satisfactoryresults. In addition, the use of the term “at least one of X, Y and Z”will be understood to include X alone, Y alone, and Z alone, as well asany combination of X, Y and Z.

As used herein, all numerical values or ranges include fractions of thevalues and integers within such ranges and fractions of the integerswithin such ranges unless the context clearly indicates otherwise. Thus,to illustrate, reference to a numerical range, such as 1-10 includes 1,2, 3, 4, 5, 6, 7, 8, 9, 10, as well as 1.1, 1.2, 1.3, 1.4, 1.5, etc.,and so forth. Reference to a range of 1-50 therefore includes 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc., upto and including 50, as well as 1.1, 1.2, 1.3, 1.4, 1.5, etc., 2.1, 2.2,2.3, 2.4, 2.5, etc., and so forth. Reference to a series of rangesincludes ranges which combine the values of the boundaries of differentranges within the series. Thus, to illustrate reference to a series ofranges, for example, of 1-10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-75,75-100, 100-150, 150-200, 200-250, 250-300, 300-400, 400-500, 500-750,750-1,000, includes ranges of 1-20, 10-50, 50-100, 100-500, and500-1,000, for example. A reference to degrees such as 1 to 90 isintended to explicitly include all degrees in the range.

As used herein, the words “comprising” (and any form of comprising, suchas “comprise” and “comprises”), “having” (and any form of having, suchas “have” and “has”), “including” (and any form of including, such as“includes” and “include”) or “containing” (and any form of containing,such as “contains” and “contain”) are inclusive or open-ended and do notexclude additional, unrecited elements or method steps.

The term “or combinations thereof” as used herein refers to allpermutations and combinations of the listed items preceding the term.For example, “A, B, C, or combinations thereof” is intended to includeat least one of: A, B, C, AB, AC, BC, or ABC, and if order is importantin a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB.Continuing with this example, expressly included are combinations thatcontain repeats of one or more item or term, such as BB, AAA, AAB, BBC,AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan willunderstand that typically there is no limit on the number of items orterms in any combination, unless otherwise apparent from the context.

Throughout this application, the terms “about” and “approximately” areused to indicate that a value includes the inherent variation of error.Further, in this detailed description, each numerical value (e.g.,temperature or time) should be read once as modified by the term “about”(unless already expressly so modified), and then read again as not somodified unless otherwise indicated in context. As noted, any rangelisted or described herein is intended to include, implicitly orexplicitly, any number within the range, particularly all integers,including the end points, and is to be considered as having been sostated. For example, “a range from 1 to 10” is to be read as indicatingeach possible number, particularly integers, along the continuum betweenabout 1 and about 10. Thus, even if specific data points within therange, or even no data points within the range, are explicitlyidentified or specifically referred to, it is to be understood that anydata points within the range are to be considered to have beenspecified, and that the inventors possessed knowledge of the entirerange and the points within the range. The use of the term “about” maymean a range including ±10% of the subsequent number unless otherwisestated.

As used herein, the term “substantially” means that the subsequentlydescribed parameter, event, or circumstance completely occurs or thatthe subsequently described parameter, event, or circumstance occurs to agreat extent or degree. For example, the term “substantially” means thatthe subsequently described parameter, event, or circumstance occurs atleast 90% of the time, or at least 91%, or at least 92%, or at least93%, or at least 94%, or at least 95%, or at least 96%, or at least 97%,or at least 98%, or at least 99%, of the time, or means that thedimension or measurement is within at least 90%, or at least 91%, or atleast 92%, or at least 93%, or at least 94%, or at least 95%, or atleast 96%, or at least 97%, or at least 98%, or at least 99%, of thereferenced dimension or measurement (e.g., length).

As used herein any reference to “one embodiment” or “an embodiment”means that a particular element, feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment.

As used herein any reference to “we” as a pronoun herein refersgenerally to laboratory personnel or other contributors who assisted inthe laboratory procedures and data collection and is not intended torepresent an inventorship role by said laboratory personnel or othercontributors in any subject matter disclosed herein.

The following abbreviations apply:

-   -   ABS: acrylonitrile butadiene styrene    -   ASR: airport surveillance radar    -   AUT: antenna under test    -   CPPAR: cylindrical polarimetric phased array radar    -   dB: decibel(s)    -   E-plane: electric field plane    -   GHz: gigahertz    -   H: horizontal    -   HFSS: high-frequency structure simulator    -   H-plane: magnetic field plane    -   m: meter(s)    -   mm: millimeter(s)    -   MPAR: multi-function phased array radar    -   PPPAR: planar polarimetric phased array radar    -   PTFE: polytetrafluoroethylene    -   SENSR: Spectrum Efficient National Surveillance Radar Program    -   TDWR: terminal doppler weather radar    -   UEAEP: unit excitation active element pattern    -   V: vertical    -   3D: three-dimensional    -   °: degree(s).

Returning to the detailed description, in one non-limiting embodiment ofthe antenna, a dual-polarized microstrip patch antenna array is designedfor multifunction radar application. A higher than 51 dB horizontal tovertical ports isolation and a better than −30 dB cross-polarizationlevel is achieved from the fabricated single element measurements. Toimprove the cross-polarization level, a 2×2-element subarray, which isconfigured according to the image feed method, is designed andfabricated, and a better than −39 dB cross-polarization level isobserved from the measurements results. The return loss and couplingbetween horizontal and vertical ports are simulated using periodicboundary conditions in CST Microwave Studio. The simulation resultsshowed that the return loss of H and V ports stay below −10 dB whilescanning up to 45°, and a less than −45 dB H to V port coupling isachieved at a 45° scan angle. Using a UEAEP method, the 4×10-elementarray antenna radiation pattern is measured at 4 different scan anglesand in the main beam area. Better than −45 dB cross-polarization levelis achieved with H-pol and V-pol excitations in both principal planeswhile scanning up 45°.

EXAMPLES

The disclosed concepts will now be discussed in terms of severalspecific, non-limiting examples. The examples described below, whichinclude particular embodiments, will serve to illustrate the practice ofthe present disclosure, it being understood that the particulars shownare by way of example and for purposes of illustrative discussion ofparticular embodiments of the present disclosure only and are presentedin the cause of providing what is believed to be a useful and readilyunderstood description of construction procedures as well as of theprinciples and conceptual aspects of the inventive concepts.

Example 1: Single Element Design

The layer stack-up and the design parameters of the proposed unit cellare presented in FIG. 1 and FIGS. 2A-2D. On the front side of the firstlaminate, the feed lines for both horizontal and vertical polarizationsare laid. To achieve the maximum bandwidth and to have the minimumsurface wave effect, it is always desired to implement a material withlow dielectric constant. However, using materials with a low relativepermittivity will increase the unit cell dimensions. Also, sincematerials with low relative permittivity, for instance, RT/duroid 5880,are based on PTFE composites, special treatment for metalized holes isrequired. In this design, an RO4534 laminate with the relativepermittivity of 3.4 and a thickness of 0.813 mm is chosen for the firstsubstrate, which contains feed lines and metalized holes for connectors.The ground plane, which includes three slots, is located on the backside of the RO4534 laminate. The radiating and parasitic patches arelocated on the back side of the second and third laminates, which are3.175 mm thick RT/duroid 5880. In the proposed design, a low dielectricmaterial, RT/duroid 5880, is used to achieve the required bandwidth formultifunction applications, and RO453 with the higher dielectricmaterial is used for reducing the size of the transmission lines andease of fabrication.

As seen in FIGS. 2A-2D, the horizontal polarization feed line and thecorresponding H-shaped slot are placed in the middle of the antenna. Thehorizontal polarization slot is symmetric with respect to horizontal andvertical planes, and it is positioned in the middle of the ground plane.

Exemplary, non-limiting, values for the various length and widthparameters shown in the embodiments of FIG. 2A-2D are shown in Tables1-3. Each parameter value may be varied by plus/minus 0.001% to 500%,for example.

TABLE 1 Exemplary antenna parameter values Parameter Value (mm)Value/Wavelength (λ) l₁  7.9 0.07373 l₂  5.65 0.05273 l₃ 11.45 0.10687l₄  8.85 0.0826 l₅ 14.7 0.1372 l₆ 29.85 0.2786 l₇  4.6 0.04293 l₈  4.20.0392 l₉  7.4 0.06907 l₁₀  9.5 0.08867

TABLE 2 Exemplary antenna parameter values Parameter Value (mm)Value/Wavelength (λ) w₁  1.7 0.0159 w₂  0.95 0.0089 w₃  0.76 0.0071 w₄ 0.6 0.0056 w₅  1.8 0.0168 w₆  0.9 0.0084 w₇  3.1 0.0289 w₈  1.5 0.014w₉ 27.7 0.2585 w₁₀ 28.9 0.2697

TABLE 3 Exemplary antenna parameter values Parameter Value (mm)Value/Wavelength (λ) l₁ 4.5 0.042 l₂ 7 0.06533 l₃ 4 0.03733 l₄ 2.70.0252 l₅ 8.4 0.0784 l₆ 6.875 0.06417 h_(sub1) 0.813 0.00759 h_(sub2)3.175 0.02963 h_(sub3) 3.175 0.02963 h_(bondply) 0.076 0.00071

Antenna Design

One aspect of the design and development of the low cross-polarizationand high-isolation patch antennas is to increase the symmetry of design.As mentioned above, the horizontal polarization slot is designed to bein the middle of the ground plane. Therefore, the only way to maintainthe symmetry of the design without having a complicated multilayerdesign is to excite the vertical polarization through a differentialfeed method. To implement the differential feeding method, two similarH-shaped slots are placed beside the horizontal polarization slot. Inthe presented differential feed method, to suppress the higher-ordermodes and reduce the cross-polarization level, the two slots are excitedwith a 180° phase shift. As seen in FIG. 1, the required 180° phaseshift for the differential feed method is produced through the lengthdifference of the two branches of the vertical polarization excitationfeed line.

The typical bandwidth of a microstrip patch antennas is 3% percent. Oneway to increase the bandwidth in the microstrip patch antenna is toincrease the substrate thickness or to use the stacked patch method.Increasing the overall thickness of the microstrip patch antenna willresult in the excitation of higher-order modes. Higher-order modesincrease the coupling between the orthogonal polarizations and degradethe cross-polarization level, especially the dual linear polarizedmicrostrip patch antennas.

The allocated bandwidth for MPAR operation when replacing ASR and TDWRis 2.7-2.9 GHz. The multilayer configuration bandwidth enhancementmethod is implemented in this design. In one non-limiting embodiment, aparasitic patch is placed on top of the radiating square patch. Forbonding three different laminates, an adhesive material such as 0.076 mmthick Rogers 2929 Bondply may be utilized. The photograph of thefabricated unit cell is shown in FIG. 3 and provides the simulated andmeasured S-parameters. FIG. 4 demonstrates agreement between thesimulation and measurement results.

Single Element Methods and Results

The S-parameters of the proposed single element are measured by using anN5225A network analyzer from Agilent Technologies.

Step 1: The network analyzer and attached cables are calibrated usingthe ECal module.

Step 2: After calibration, the S-parameters were measured in a smallanechoic chamber to ensure that interference of unwanted radiation andreflection are minimized.

Step 3: The measured S-parameters are then exported and plotted alongwith the simulation results.

For horizontal and vertical polarizations, below a −12.1 dB return losshas been achieved from simulated and measured results in the entirebandwidth (2.7-2.9 GHz). Also, the horizontal and vertical polarizationreturn loss results are similar, which decreases the gain mismatchbetween the two polarizations. As seen in FIG. 4, the isolation betweenpolarizations is better than 52 dB. To measure such low coupling betweenports, the S-parameter measurements are conducted in shielded anechoicchambers designed for S-parameter measurements. As seen in FIG. 4, wemanaged to measure a higher than 51 dB input isolation in the entirebandwidth.

FIG. 5 shows the S-parameters versus scan angles in φ=0° and φ=90°planes. As seen in FIG. 5, at the MPAR operating frequency, thesimulated return loss results stay below −10 dB while scanning up to 45°in both principal planes. The isolation between two orthogonalpolarizations in the required scanning range is better than 45 dB.

The single element radiation pattern is measured in a far-field anechoicchamber.

Step 1: The single element has been precisely aligned with the standardmeasurement probe.

Step 2: Horizontal polarization radiation patterns are measured whilethe vertical polarization was terminated. The co-polarization andcross-polarization patterns have been measured by changing thepolarization of the measurement probe. The radiation patterns of thesingle element in φ=0° and φ=90° have been measured by rotating thesingle element 90°.

Step 3: Vertical polarization radiation patterns are measured while thehorizontal polarization was terminated. The co-polarization andcross-polarization patterns have been measured by changing thepolarization of the measurement probe. The radiation patterns of thesingle element in φ=0° and φ=90° have been measured by rotating thesingle element 90°.

The measured radiation pattern of the fabricated hybrid feed patchantenna is provided in FIGS. 6A-6D. The measured cross-polarizationpatterns of horizontal polarization in principle planes are presented inFIG. 6A and FIG. 6B. For the horizontal polarization, the single elementcross-polarization level at 2.8 GHz is below −36 dB in φ=0° plane andbetter than −35 dB in φ=90° plane. As seen in FIG. 6C and FIG. 6D, themeasured cross-polarization level while vertical polarization is excitedat 2.8 GHz is better than −30 dB in φ=0° plane and less than −36 dB inφ=90° plane. Although this level of cross-polarization is very low, tosatisfy MPAR requirements, polarization purity of higher than 40 dB isrequired.

Example 2: Cross-Polarization Suppression and Subarray Design

A dual-polarized antenna requires two individual ports for excitingorthogonal polarizations, and the low cross-polarization level isdesired. A differential feeding technique will suppress the higher-ordermodes and all cross-polarization components of the antenna radiationpattern. A microstrip patch antenna with dual polarization functionalityis realized while each polarization is excited with two 180° out-phaseports, which is called an ideal differential feed patch antenna. Theadvantage of using the ideal differential feed patch antenna is itsextremely low cross-polarization level, especially in the principalplanes. However, an ideal differential feed requires external 180° phaseshifters. In a phased array radar, the increased quantity of connectors,cables, and phase shifters would significantly increase fabricationcosts. An alternative solution for reducing the cross-polarization levelis to arrange the elements of the array into the groups of 2×2-elementidentical subarrays in which the horizontal polarization ports aremirrored. The present disclosure describes the method of improving thecross-polarization level. A photograph of the designed subarrayconfigured according to the image configuration is shown in FIG. 7.Similar to an ideal differential feed antenna, a 180° phase differenceis applied for exciting the mirrored ports.

One non-limiting embodiment utilizes an integrated balun to excite onepolarization at two locations with a 180° phase shift. In this design,the signal at the end of the feeding lines of the balun will be 180° outof phase from each other. The length of the transmission lines isadjusted to provide a 180° phase difference. The required length betweenthe two arms of the vertical polarization transmission line iscalculated analytically and then optimized in Ansys HFSS. The two 180°out-of-phase signals at the end of the two ends of the balun will becoupled to the radiating square patch through two apertures. Having two180° out-of-phase excitation sources for one polarization will suppressthe higher-order modes and cross-polar components of the antennaradiation pattern.

Methods and Results

The subarray radiation pattern was measured in a far-field anechoicchamber. The subarray is made of a 2×2 element array of the antennas,while the vertical polarization is mirrored with respect to thehorizontal plane.

Step 1: The center of the subarray has been precisely aligned with thestandard measurement probe.

Step 2: To measure the subarray horizontal polarization radiationpattern, a 4-way power divider has been implemented to excite all fourin-phase ports. The co-polarization and cross-polarization patterns havebeen measured by changing the polarization of the measurement probe. Theradiation patterns of the subarray in φ=0° and φ=90° have been measuredby rotating the subarray for 90°.

Step 3: To measure the subarray vertical polarization, three 2-way powersplitters and phase shifters were used, and two mirrored ports wereexcited with a 180° phase shift with respect to the two other ports. Theco-polarization and cross-polarization patterns have been measured bychanging the polarization of the measurement probe. The radiationpatterns of the subarray in φ=0° and φ=90° have been measured byrotating the subarray 90°.

The measured radiation patterns of the fabricated 2×2-element subarrayof the designed unit cell in FIG. 7 are shown in FIGS. 8A-8D. Accordingto the measurement results, for both polarizations in the E-plane andH-plane from 2.7 GHz to 2.9 GHz, the cross-polarization level is around−40 dB. At the center frequency, with H-pol excitation thecross-polarization level is better than −40 dB in φ=0° and less than −41dB in φ=90° plane. For vertical polarization at 2.8 GHz, the maximumcross-polarization level is less −37 dB in φ=0° planes and less than −41dB in φ=90° plane. Also, the simulated cross-polarization level for bothprincipal planes is better than −51 dB. The discrepancy betweensimulated and measured cross-polarization level is the result of unidealmeasurement environments such as cross-polarization of the transmittingantenna and backscattering of the antenna cable and positioner andpossible fabrication errors.

Example 3: Array Design

To characterize the scan radiation pattern of the unit cell andsubarray, a 2×5-element array of the presented subarray is fabricated.The geometry of the fabricated 4×10-element array, which is made forcharacterizing the scan characteristics of the unit cell at differentscan angles in the φ=0° plane, is shown in FIG. 9A. For measuring lowcross-polarization levels, the alignment of the AUT with a transmitterantenna plays a key role. Considering perfect conditions in the anechoicchamber, any misalignment between the AUT and transmitter antennaresults in measuring the cross-polarization level in off-principleplanes. For a perfect alignment between the AUT and the transmitterantenna, the antennas are installed on the fixture, which is fabricatedfrom plexiglass. These plexiglass components of the antenna fixture areprecisely processed by a laser cutting machine. The two white componentsof this fixture are made from ABS by using a 3D printer. As seen in FIG.9B, to characterize the array scanning performance in φ=90° plane, the2×2-element subarrays are rotated 90°.

Although a 4×10-element array antenna is fabricated for characterizingthe performance of the designed single element, for MPAR applications,final array dimensions may be as large as a cylindrical array antennawith a 10 m diameter. Therefore, to decrease the edge element's effecton the array radiation characteristics, one element from each side isterminated. The simulated realized gain of the proposed unit cell at 2.8GHz is 6.7 dB and 6.8 dB with the H-pol and V-pol excitations,respectively. With the 2×8-element array configuration, the simulatedrealized gain increases to 17.26 dB and 16.88 dB for horizontal andvertical polarizations, respectively.

Methods and Results

The array antenna radiation patterns are measured according to the UEAEPmethod.

Step 1: Every element pattern is measured separately, while all otherremaining elements were terminated.

Step 2: Measure the co-polarization and cross polarization radiationpattern in φ=0° and φ=90° planes.

Step 3: The magnitude and phase of all the measured active elementpatterns are imported into MATLAB.

Step 4: The required phase shift between elements to steer the arrayradiation pattern is calculated and applied to the measured radiationpattern. The active reflection coefficient magnitude of the entirelyexcited antenna array at the steering angles is contributing to themeasured realized gain while measuring the active element pattern.Therefore, using the UEAEP method for characterizing the array scanradiation pattern decreases the cost and risk of failure in themeasurements of the prototypes.

Following the UEAEP method, the array antenna measured scan pattern inprinciple planes at the MPAR operating frequency are shown in FIGS.10A-10B and FIGS. 11A-11B. FIGS. 10A-10B show measured scan radiationpatterns of the central 2×8-element array in the 4×10-element array ofFIGS. 9A-9B at 2.7 GHz, 2.8 GHz, and 2.9 GHz. FIGS. 11A-11B showmeasured scan radiation patterns of the central 2×8-element array in the4×10-element array of FIGS. 9A-9B at 2.7 GHz, 2.8 GHz, and 2.9 GHz.Also, the array element's excitation amplitude is adjusted according to25 dB Taylor amplitude tapering to decrease the sidelobe level. With theH-pol excitation, the array cross-polarization level while scanning up45° remains less than −40 dB in the φ=0° plane. Also, thecross-polarization level of less than −44 dB is achieved in the φ=90°plane with the H-pol excitation. It is seen that the cross-polarizationlevels of V-pol excitation are better than −40 dB in the φ=0° plane and−39 dB in the φ=90° plane with scanning up to 45°. The reportedcross-polarization values are the peak of the cross-polarization at 2.8GHz from −90°<0<90°. For the scanning up to 20°, which is the maximumrequired beam steering for cylindrical geometry, the cross-polarizationlevels in the main beam area are mostly below −45 dB. This level of thecross-polarization pattern could satisfy the MPAR requirements.

While the present disclosure has been described in connection withcertain embodiments so that aspects thereof may be more fully understoodand appreciated, it is not intended that the present disclosure belimited to these particular embodiments. On the contrary, it is intendedthat all alternatives, modifications and equivalents are included withinthe scope of the present disclosure. Thus the examples described above,which include particular embodiments, will serve to illustrate thepractice of the present disclosure, it being understood that theparticulars shown are by way of example and for purposes of illustrativediscussion of particular embodiments only and are presented in the causeof providing what is believed to be the most useful and readilyunderstood description of procedures as well as of the principles andconceptual aspects of the presently disclosed methods and compositions.Changes may be made in the structures of the various componentsdescribed herein, or the methods described herein without departing fromthe spirit and scope of the present disclosure.

What is claimed is:
 1. A microstrip patch antenna comprising: a unitcell comprising: a plurality of layers comprising: a first laminatecomprising one or more horizontal polarization feed lines and one ormore vertical polarization feed lines, a second laminate comprising aradiating square patch, and a third laminate comprising a parasiticpatch; and a ground plane comprising one or more polarization slots. 2.The microstrip patch antenna of claim 1, wherein the plurality of layersfurther comprises an adhesive material positioned between the firstlaminate and the second laminate and between the second laminate and thethird laminate.
 3. The microstrip patch antenna of claim 1, wherein theparasitic patch is positioned adjacent to the radiating square patch. 4.The microstrip patch antenna of claim 1, wherein the plurality of layersfurther comprises metalized apertures.
 5. The microstrip patch antennaof claim 1, wherein the one or more polarization slots comprise ahorizontal polarization slot positioned symmetrically with respect to ahorizontal plane and a vertical plane.
 6. The microstrip patch antennaof claim 5, wherein the horizontal polarization slot is positioned in amiddle of the ground plane.
 7. The microstrip patch antenna of claim 1,further comprising one or more horizontal polarization ports, whereinthe one or more horizontal polarization ports are offset.
 8. Themicrostrip patch antenna of claim 7, further comprising one or morevertical polarization ports orthogonal to the one or more horizontalpolarization ports.
 9. The microstrip patch antenna of claim 1, whereinthe one or more horizontal polarization feed lines are mirrored.
 10. Amicrostrip patch antenna subarray comprising: a plurality of unit cells,each comprising: a first laminate comprising one or more horizontalpolarization feed lines and one or more vertical polarization feedlines, a second laminate comprising a radiating square patch, a thirdlaminate comprising a parasitic patch, and a ground plane comprising oneor more polarization slots.
 11. The microstrip patch antenna subarray ofclaim 10, further comprising: one or more horizontal polarization portsoffset from each other; and one or more vertical polarization portsorthogonal to the one or more horizontal polarization ports.
 12. Themicrostrip patch antenna subarray of claim 10, wherein the one or morehorizontal polarization feed lines are mirrored.
 13. The microstrippatch antenna subarray of claim 10, wherein each of the plurality ofunit cells further comprises an adhesive material positioned between thefirst laminate and the second laminate and between the second laminateand the third laminate.
 14. The microstrip patch antenna subarray ofclaim 10, wherein the parasitic patch is positioned adjacent to theradiating square patch.
 15. The microstrip patch antenna subarray ofclaim 10, wherein each of the plurality of unit cells further comprisesmetalized apertures.
 16. The microstrip patch antenna subarray of claim10, wherein the one or more polarization slots comprise a horizontalpolarization slot positioned symmetrically with respect to a horizontalplane and a vertical plane.
 17. A differential feed antenna comprising:a balun; a plurality of feed lines; and one or more polarization portsconfigured to excite at a plurality of locations.
 18. The differentialfeed antenna of claim 17, wherein the balun is configured to receive oneor more signals from the plurality of feed lines.
 19. The differentialfeed antenna of claim 18, wherein the one or more signals comprise afirst signal and a second signal that are 180° out of phase from eachother.
 20. The differential feed antenna of claim 17, wherein each ofthe plurality of feed lines is configured to suppress high-order modes.